I had a toy like this growing up:http://www.toystogrowon.com/sku185
I loved it so much, I am pretty sure I still have it somewhere! It was so much fun to see how the parts connected and where the marble would come out. I’m more for this kind of thing than a passive toy on the wall, cool as it may be.

I remember there being a larger version (biggger spheres) using paths across the ceilings and through the walls of the top of one of the Trade Center towers – where the cafe and gift shop(s) were. These types of fun, along with slot cars and train sets make me a kid again easily. Perhaps I never strayed too far from that territory.

At about 19 seconds in, you can clearly see a shift register implemented using marbles and wooden channels. We could likely generalize this up to something that operates much like a camera’s CCD, but the full operation of the system is visible to the naked eye. This might be a very nice instructive tool to show anyone how digital cameras work.